Frequently, broadband systems transmit television signals and programs to subscribers of a conditional access system. Broadband systems, such as cable and satellite television systems, typically include a headend for receiving programming and/or data from various sources and redistributing the programming and other data through a distribution system to subscribers. The headend receives programming signals from a variety of sources, combines the programming signals from the various sources, and transmits the combined signals through the distribution system to subscriber equipment. The distribution system can include a variety of media, such as coaxial cable, fiber optic cable, and satellite links among others. In a subscriber television system, the subscriber equipment, which receives the signals from the headend, can include, among others, a cable-ready television, a cable-ready video cassette recorder (VCR), or a digital subscriber communications terminal (DSCT) that is connected to a television, computer, or other display device.
The headend uses modulators to control the streams of data into the distribution system. Increasingly, the headend is receiving and transmitting programming in a digital format, for example, Moving Pictures Expert Group (MPEG) format, instead of an analog format. Transmitting programs in MPEG format is advantageous because multiple digitized programs can be combined and transmitted in, for example, 6 MHz of bandwidth, which is the same amount of bandwidth that is required to transmit a single analog channel or program, and in comparison to analog programs, MPEG or digitized programs provide a cleaner and sharper image and sound. Various error correction schemes enable the digital packets to be transmitted through a digital network with minimal distortion or error.
In order to thwart unauthorized access to the content of the broadband system, the content is usually encrypted at the headend prior to distribution. The headend provides the authorized subscribers of the broadband system with the keys necessary to decrypt the encrypted content. Typically, content such as programs or instances of service are encrypted using a symmetrical cryptographic algorithm. A symmetrical cryptographic algorithm uses a pair of functions (F and F−1) and a single key (k) or keys for both encryption and decryption. When a function, F or F−1, is applied to cleartext (C) using a key (k), the cleartext is converted to ciphertext (C′). The produced ciphertext, C′, depends upon the cleartext, C, the key, k, and upon which function, F or F−1, was used to produce it. For example:                F(k; C)=C′{F; k},        F−1(k; C)=C′{F; k}, and        C′{F; k}≠C′{F−1; k}.Similarly, ciphertext produced from the same cleartext and the same function, G=F or F−1, but with different keys k1 and k2, are different:        G(k1; C)≠G(k2; C).        
Ciphertext can be converted back into cleartext by using the appropriate function with the appropriate key. The appropriate function being the function that is the inverse of the function used for generating the ciphertext. Either function can be used for converting cleartext to ciphertext, and when the same key is used, each reverses the operation performed by the other, e.g.,                F(k; F−1 (k; C))=F−1(k; F (k; C))=C.        
The function F is conventionally referred to as the encryption function (E) and the function F−1 is conventionally referred to as the decryption function (D). However, this conventional naming scheme can be confusing, because the decryption function can be used for encryption, i.e., converting cleartext to ciphertext, and the encryption function can be used for decryption, i.e., converting ciphertext to cleartext.
Some cryptographic algorithms such as 3DES use multiple functions and keys to convert between cleartext and ciphertext. In one embodiment of 3DES encryption, cleartext is converted to ciphertext according to the following scheme:                F(k3; F−1 (k2; F (k1; C)))=C′″{F, F−1, F; k1, k2, k3}.The first operation, F(k1; C), produces ciphertext C′(F; k1). The single prime (′) designates that the ciphertext C′{F; k1} has one layer of encryption thereon. The second operation, F−1 (k2; F (k1; C)) or F−1 (k2; C′ {F; k1}), applies the function F−1 using the k2 on the ciphertext C′{F; k1} to produce the ciphertext C″{F, F−1; k1, k2}, thereby applying a second layer of encryption. The second operation does not produce cleartext, C, because different keys are used, e.g., k1 does not equal k2. The third operation, which adds a third layer of encryption, applies the function F using the key k3 on the C″ {F, F−1; k1, k2} to produce the ciphertext C′″ {F, F−1, F; k1, k2, k3}. To convert C′″ {F, F−1, F; k1, k2, k3} back to cleartext C, the inverse functions F and F−1 must be applied with the appropriate key in reverse order:        F−1 (k1; F (k2; F−1 (k3; C′″ {F, F−1, F; k1, k2, k3})))=C.        
In an alternative implementation of the 3DES cryptographic algorithm, the first key and third keys are the same, k1=k3. In that case, cleartext is converted to ciphertext according to the following scheme:                F(k1; F−1 (k2; F (k1; C))) C′″{F, F−1, F; k1, k2, k1}.        
In theory, the packets of a digital program can be reproduced or copied without error. Thus, a subscriber of a subscriber network who receives a digital program can record the program and copy it, and the copy will be virtually identical to the original. Therefore, there exists concern about illegal copying or bootlegging of digital content. The operators of a subscriber network and the content providers want to provide the subscribers of the digital network with the programming and services desired by the subscribers, but the digital content owners want to prevent the subscribers from making and distributing bootleg copies of the digitized programs and services. Thus, there exists a need for an apparatus that protects the property interests of the digital content owners, while providing the subscribers with the desired digital content.